Aluminum-alloyed lightweight stainless steels strengthened by B2-(Ni,Fe)Al precipitates

Abstract

The age hardenability of Al-alloyed lightweight stainless steels with the base chemical composition Fe–10.5Cr–3Al (wt.%) and Ni concentrations in the range 3–15 wt.% was studied. Alloys containing 3% and 6% Ni exhibited almost fully ferritic matrix microstructures and a weak age hardening response. Alloys containing 9%, 12%, and 15% Ni, on the other hand, developed primarily martensitic microstructures. Differential scanning calorimetry measurements indicated the occurrence of an exothermic reaction in the approximate temperature range 375–625 °C. Dilatometry measurements indicated that the exothermic reaction was accompanied by a net contraction. Hardness measurements after aging for 5 min indicated significant hardening of alloys already at 350 °C due to the formation of B2-(Ni,Fe)Al intermetallic precipitates. The age hardening response was significantly superior to that of conventional precipitation-hardenable martensitic stainless steels. Tensile elongation in the aged condition was negatively influenced by the presence of soft ferrite regions. Processing conditions associated with a fully martensitic microstructure prior to aging are required to render a uniform age hardening response. Guidelines for the development of a new family of lightweight precipitation-hardenable steels with lower raw material costs and a higher corrosion resistance compared to the standard 18Ni maraging steels are provided.